Study On Quasi-commutative Watermarking Algorithm For Encrypted Image

With the rapid development of information technology and computer networks,digital media has been widely spread. It provides convenience for people to exchangeinformation, but at the same time it also brings many security issues. A variety ofmultimedia information can be illegally copied, modified, destroyed and distributed. Sothe security problems of digital products have caused more and more attention. As onemajor carrier of multimedia information, digital image is used widely with its fresh andclear and vivid characteristics. Therefore, how to protect the security of digital imageneeds to be resolved urgently.In recent years, there have been many techniques about secure digital image, inwhich encryption and watermarking are two important branches with many researchresults. Now most researchers adopt either of the two approaches to guarantee thesecurity of information transmission. While in the real-world applications, someproblems need the combination of encryption and watermark technology.Generally, to realize the combination of encryption and watermark technology，there are two steps. Firstly, media data are watermarked. Secondly, the watermarkedmedia data are encrypted. In this case, media data must be decrypted before anotherwatermark can be embedded. That is, if you want to embed the watermark into theencrypted media, you need to do the decryption–watermarking–encryption triples.Just under this background, this thesis studies the overseas and domestic researchstatus, analyses the main image encryption algorithms and the reversible watermarkingalgorithms, grasps the technical principles of present cryptography and reversiblewatermarking and tries to find out joint point of them.The main work of this thesis is as follows:①The related knowledge about digital watermarking and encryption technique isintroduced, followed by the feature and application background of watermarking in theencrypted domain;②The homomorphic encryption scheme is analysed in details. This scheme canallow people to process digital media under the cipher condition;③The features、category and existing popular embedding schemes of reversiblewatermarking are introduced in details, followed by the assessment standards ofwatermarking schemes. ④To meet the requirement of reversibly embedding watermarking in the encrypteddomain, a novel reversible watermarking algorithm in histogram difference domain forencrypted image is proposed by utilizing a combination of histogram shifting andhomomorphic encryption. This algorithm has the quasi-commutative property. It candirectly embes watermark into the plain image or the encrypted image without theoriginal image information. That is a process of "original image-encryption-watermarkembedding" which can simplify the traditional process of "originalimage-encryption-decryption-watermarkembedding-encryption".Simulationexperimental results demonstrate that the proposed algorithm can provide greaterpayload capacity and higher image fidelity. After it extracts the watermark completely,it can recover the original image reversibly. It outperforms other related algorithms.Finally, the conclusions of this thesis are given and the future direction is alsodiscussed.